As automated assembly equipment and robots become sophisticated, it is desirable to add the sense of vision to such devices. This would enable the equipment to locate the position of an object being manufactured, as well as to inspect the object for the presence of components or the proper location and size of specific features. To this end, various vision systems have been employed to generate a one or two dimensional electrical image of the workpiece which then can be electronically analyzed.
Conventional charge coupled device (CCD) image sensors have frequently been used in these vision systems. A CCD imager consists of a one or two dimension array of detector elements which accumulate electrical charge in proportion to the intensity of light striking them. After the charge is allowed to accumulate for a period of time, the detector elements are sequentially scanned and a video signal is produced in response to the charge in each element. The resultant video signal represents an image to be analyzed.
A certain amount of charge will accumulate in each detector element even when light does not strike that element. When such a detector element is scanned, a "dark current" is produced corresponding to the intrinsic charge accumulation. The dark current is a function of temperature, the period that charge was allowed to build up in each detector element and the electrical characteristics of the image sensor. Even detector elements exposed to light will have a portion of their accumulated charge due to the intrinsic charge buildup.
A common image analysis technique processes picture elements of the image which have a luminance above or below a set threshold level. For example, very bright picture elements having a luminance above a relatively high threshold may be counted and the sum used as an analysis parameter. In this technique, the dark current contribution to the video signal from the CCD image sensor can have a significant impact on the image analysis, as it affects the relative magnitude of the video signal luminance.